Thursday, February 12, 2015

Merely a flesh wound: Lizard autotomy and regeneration

Imagine you’re a small animal that has been pinned down by a predator. You are essentially doomed.

Let go, you lazy beast!

Unless of course, you can shed a relatively large body part that thrashes and wiggles about to distract the predator—knowing that in a few months you will have replaced the body part entirely!

Come and get me!

Many lizards are prepared for this exact situation, and are able to sever their tails from their body. This type of shedding of body parts is known as autotomy, and can occur in insects and salamanders, but today I’m going to focus on lizards.

This topic may be familiar to many—think back on younger days when you went chasing after a lizard that looked like this guy ---------->

And when you caught the fella, all you got away with was this:

Ah! Duped again!

Blue-tailed skinks (and many other lizards with conspicuously colored tails) are common to multiple continents, so scientists wanted to investigate the possible evolutionary benefit of this. Bateman et. al 2014 created clay lizard models with blue and brown tails, and left them out in the environment to see if the models were attacked differently by avian predators.

Y axis is % of days damage was recorded on the models. Blue-tailed models were attacked more overall, and attacks were targeted on their tails.

They found that blue-tailed models more frequently attacked on tails than the brown models. However, the blue-tailed models were more frequently attacked overall—BUT since the attacks were tail-directed, the attacks would not be fatal to a real lizard, which could simply autotomize its tail and get away. The brown modeled lizards sustained more fatal, head-directed injuries, which suggests that the flashy blue tail either prevents predators from zeroing in on the head or is somehow more appealing. This risky-decoy defense tactic has been fascinating scientists (and apparently predators!) for decades.

Maybe I can't regenerate like I used to...

Even more fascinating, perhaps, is how lizards can autotomize and regenerate their tails in the first place! Not all lizards that autotomize have flashy blue tails. In fact, adult skinks of the same species will lose their blue tail coloration as they mature, and some scientists believe this has to do with regeneration capability differences in juvenile vs. adult lizards.

The jury is still out on how the tail is severed off, and what conditions must occur to cause muscle contractions that can sever the nerves, bone, and corresponding connecting tissue with such a clean break. Lizards have cool adaptations to help facilitate autotomy and prevent trauma that many other animals would experience after tearing off a body part!

Everything below the arrow is a tail full of fracture planes.

Lizard tails have fracture planes—which are essentially segments where the tail can break off. In fact, legless lizards can break their tail into so many pieces due to fracture planes they are known as "glass lizards"! The planes are between vertebrae, and are accompanied by fibrous fracture planes and also separate the myomeres, or muscle segments. The arteries in the tail are squeezed shut by specialized sphincters to prevent blood loss, so the lizards can make a clean getaway!

A typical caudal segment, showing the "tears" in muscle fibers.

The predator *theoretically* becomes distracted by the twitching tail, which can jump and thrash about for several minutes. The two general types of tail movement are ballistic (jumping) and rhythmic (swinging) movements.

An example of ballistic tail movement:

Higham et. al (2014) hypothesize that different muscle fiber types in tail are responsible for each twitch type. There is a superficial layer of slow-twitch, oxidative muscle fiber in the tail, which may be responsible for the rhythmic swinging movement of a tail, as this motion tends to outlast ballistic behavior.

An example of rhythmic tail movement:

The fast twitch, less oxidative muscle fibers compose the majority of the tail. These fibers are likely responsible for the short-lived but ballistic movements seen in some autotomized tails.

While the predator celebrates its victory in catching a twitchy meal, the lizard escapes and will eventually regenerate its lost body part.

Well, maybe not as quickly as that... but you get the point.

Not all lizards are capable of regenerating to the same capacity, but in those that do, the process begins with re-epithelialization of the wound site, followed closely by new epidermal growth to replace the skin and scales. Underneath, a blastema forms, which is an accumulation of mesenchymal-like tissue. This means the blastema is full of undifferentiated cells that are able to develop into different tissue types to facilitate regrowth of arteries and veins, muscles, and cartilage. The new tail is also innervated by growth of ganglia from the original spinal cord, and an ependymal tube forms in place of a true spinal cord in the regenerated tail. It is thought that this tube is formed from remnant cells from the neural tube of the developing embryo, but this is uncertain.

Here you can watch a time-lapse of the regeneration process, set to music:

Original tails reach a higher maximum velocity while twitching!

The regenerated tail very much resembles the original, and can function to aid predator escape by autotomizing AGAIN! That is, if there is still some original tail left with fracture planes to induce a second autotomy (see below). A study investigating twitch differences found that twitch duration was similar in original and regenerated tails (Meyer, et. al 2002), however, the lactate levels were lower in regenerated tails. This evidence supports a more recent study that found intensity of twitching was lessened in regenerated tails, suggesting that regenerated tissue is less effective at producing ATP to drive lactic acid metabolism during tail twitching (Naya et. al 2007). Taken with the evidence of the two tissue types driving tissue types, it is likely that the fast-twitch ballistic movement fibers are less effective in regenerated tails, but they are still able to utilize the oxidative slow twitch fibers to continue rhythmic, swinging movements.

Regenerated tails are composed of cartilage, not bone, so fracture planes are not regenerated.

So next time you try to catch a lizard and end up with a tail-- just think about the evolutionary and physiological implications of twitchy tails we still have to explore!